In-tank Fuel Pumps
Bruce Buchanan says that the in-tank pumps were fitted to 928s delivered to hot-climate markets to prevent cavitation of the
main fuel pump in hot weather.
My Australian-delivered '80 928 does not have an in-tank pump, although it has two separate wires connected to each side of the main pump, so it may
have had one which was removed when it failed, and the wires attached to the place where they were secure and couldn't do any harm.
Cavitation occurs when fuel vaporises in the fuel pump; this would occur on the inlet side of the pump (in the wake of the impeller vanes), when the pressure is low enough and the temperature is high enough for the fuel to vaporise. The fuel vaporises as bubbles, so the impeller is subject to alternating stresses as bubbles form and disappear. This manifests itself as noise, ie a high-frequency vibration which will greatly accelerate the fatigue of metal components.
The function of the in-tank pre-pump is to keep the inlet pressure at the main pump high enough that vaporisation cannot occur, even at the
elevated temperatures a foot or so above our bitumen roads in summer.
When the in-tank pump fails, however, the main pump must draw fuel through it. Happily, the main pump is up to the job for all but high fuel flows
(high load, high rpm) and its failure usually goes unnoticed (unless the failed pump sheds debris into the main pump).
The significant pressure drop across a failed pre-pump greatly increases the risk of cavitation, however, even in cooler weather. The main pump has
to work much harder and will fail sooner.
A failed in-tank pump, therefore, is worse than having no in-tank pump.
Glenn Evans '80 928 Petrolblaumetallic 5 speed Sydney